The aim of the studies described in this thesis was to develop a vaccine against feline Leukaemia (FeLV) based on inactivated virus. Previous attempts to produce an economical, inactivated virus vaccine which would protect kittens against FeLV infection had not been successful. In the present study a vaccine was prepared which protected over 80% of weanling kittens against a viral challenge which induced persistent FeLV infection in 80-100% of unvaccinated kittens. Among the reasons for the comparative success of the vaccine described here were believed to be: 1. The use of FeLV of subgroup A produced in large quantities by the F422 Lins of feline Lymphosarcoma cells. 2. Inactivation of the virus with a concentration of paraformaldehyde which preserved or enhanced the immunogenicity of the antigen. 3. The use of AL(OH)3 Freund's incomplete adjuvant in the vaccine. 4. A schedule for vaccination and challenge by FeLV which allowed the efficacy of a vaccine to be assessed clearly. In this thesis, the General Introduction is a review of the biology of FeLV and of previous attempts to produce vaccines using FeLV antigens. Chapter 1 describes the general materials and methods used throughout the study. In Chapter 2 the conditions for the preparation of FeLV for use in vaccination are described. Chapter 3 describes the inactivation of FeLV pseudotypes of murine sarcoma virus [MSV(FeLV)] with several chemicals. Chapter 4 reports the first vaccination experiment in which FeLV preparations inactivated with PF, AEI or DTNP and PF were compared as vaccines. An innocuous dose of saponin was determined in Chapter 5. The dose of saponin which was harmless was 100 ug per cat, compared to 150Q ug which had been used previously in Chapter 4. Chapter 6 deals with the preparation and determination of an appropriate challenge dose of FeLV. Chapter 7 contains the results of the second vaccination experiment, incorporating the improvements described above, in which a comparison of adjuvants was done, using PF-inactivated FeLV. Chapter 8 contains the result of the final vaccination experiment in which a comparison of antigens was done. The results in Chapter 9 are from experiments to attempt to assess in vitro the antigenicity of FeLV exposed to four types of chemical inactivating agents (Pf, Formaldehyde, AEI and betapropiolactone CBPL3). Persistence of FeLV neutralising antibodies in naturally infected cats was studied in Chapter 10. These results appeared to show that the titre of FeLV neutralising antibodies is maintained in cats for a period of over one year whether or not the cats are continuously exposed to cats excreting FeLV. These results indicate that following contact infection a cat becomes a low or a high responder. Chapter 11 is a general discussion which reviews the results in this thesis and compares the results with those of previous experiments by others. Suggestions for future work are proposed. (Abstract shortened by ProQuest.).